feat: implement exchange rate service with spread calculationFeat/idr rate aggregator

[mikleo04]

🚀 Exchange Rate Service Implementation

📌 Overview

This pull request implements a Spring Boot service that fetches exchange rate data from an external API and calculates a spread factor based on the GitHub username.

---

⚙️ Features

• Fetch latest exchange rates (IDR base)
• Fetch historical exchange rates (IDR to USD)
• Fetch available currencies
• Calculate spread using username-based formula
• In-memory data storage
• Retry mechanism and error handling for external API
• Unit and integration tests included

---

🧮 Spread Calculation

GitHub Username: mikleo04

Formula:
spread = (sum ASCII username % 1000) / 100000

Result:
0.00741 (~0.741%)

---

🏗️ Architecture

• Strategy Pattern for handling different resource types
• FactoryBean for RestTemplate configuration
• ApplicationRunner for data initialization at startup

---

🧪 Testing

• Unit tests for service, client, and datastore
• Integration tests for:
  • External API
  • ApplicationRunner startup behavior

---

📡 External API

Uses:
https://api.frankfurter.app

---

✅ Notes

• Project is fully runnable
• All endpoints tested via cURL
• README includes setup, usage, and detailed explanation